The use of high-purity, low-OH vitreous silica enables the production of optical waveguide fibers having excellent optical transmission properties, typically about 1 dB/km, in the wavelength region between 1.0-1.7 micrometers (the "transmission window") and in other regions of the far red and near infrared spectrum (See: Hetherington, G. et al., Phys Chem Glass Journal, p. 130, 1969). The main restriction heretofore encountered has arisen from optical absorption bands (See: Keck, D. B., et al., "On the ultimate laser limit of attenuation in glass optical waveguides", Appl. Phys. Letters, vol. 22, No. 7, pp. 307-309, Apr. 1, 1973; Stone, J. et al., "Overtone vibrations of OH groups in fused silica optical fibers, J. Chem Physics, 76, pp. 1712-1722, 1982) caused by hydroxyl (OH). One such absorption band occurs at 1.4 micrometers, that is, in the middle of the above-defined transmission window.
It has been observed that molecular hydrogen (H.sub.2) permeates vitreous silica (See: Hartwig, C. "Raman scattering from hydrogen and deuterium dissolved in silica as a function of pressure", J. Applied Phys., Vol. 47, page 956, 1976). Further, it has recently been reported that H.sub.2 permeation also occurs in optical fibers made out of low-OH vitreous silica and even fully clad, finished waveguide cables under certain operational conditions. (See: Mochizuki, K. et al., "Transmission loss increase in optical fibers due to hydrogen permeation", Electron. Lett., Vol. 19, No. 18, pp. 743-745, 1983; Fox, M. et al., "Attenuation changes in optical fibers due to hydrogen", Electron. Lett., Vol. 19, No. 22, pp. 916-917, 1983; Beales, K. J., "Increased attenuation in optical fibers caused by diffusion of molecular hydrogen at room temperature", Electron. Lett. Vol. 19, No. 22, pp. 917-919, 1983). The H.sub.2 permeation has been found especially prevalent when water is in contact with the fibers and/or the metal parts of the cables (See: Uesugi, N. et al., "Infra-red optical loss increase for silica fiber in cable filled with water", Electron. Lett. Vol. 19, No. 19, pp. 762-763, Sept. 15, 1983.) The presence of molecular hydrogen gives rise to a group of absorption bands between 1.0-1.2 micrometers which deteriorates the transmission properties in this spectral region.
Furthermore, it has been reported that molecular hydrogen (H.sub.2) reacts with existing defects in high purity low-OH vitreous silica, (See: Shelby, J. E., "Reaction of hydrogen with hydroxyl-free vitreous silica", J. Appl. Phys., 51 (5), pp. 2589-2593, May 1980; Friebele, E. J. et al., "Fundamental defect centers in glass: the peroxy radical in irradiated high purity fused silica", Phys. Rev. Lett Vol. 42, pp. 1346-1349, 1979) which are introduced during the fabrication process. These existing defects have been identified as peroxy defects, eg, local deviation in the glass structure caused by an extraneous oxygen atom inserted into the common Si--O--Si link changing it into Si--O--O--Si link. This defect will be herein referred to as a "peroxy link". The reaction of molecular H.sub.2 with peroxy links leads to the formation of additional OH. Thus, a low OH content material typically having 5 ppm OH, is altered to typically 80 ppm OH, (See Shelby, J. E. supra). The newly formed hydroxyls lead to a substantial increase in the optical attenuation in the spectral region between 1.4-1.65 micrometers and in other regions wherever the OH absorption bands occur. Thus the original intent of producing and maintaining low OH optical waveguide fibers is defeated.
One approach recently described by Hen-Tai Shang et al. (See: Electronics Letters, Vol. 19, No. 3, pp. 95-96, Feb. 3, 1983) comprises effecting an OH.fwdarw.OD exchange process in the substrate tube used for chemically depositing specific layers upon the undrawn fiber rod and eliminate the substrate take as a source of OH-contamination. But this technique does not eliminate the danger that the finished fiber will become permeated with H.sub.2 which thereafter slowly reacts with the peroxy linkages contained therein to produce undesired OH.
Shelby et al (See: "Radiation-induced isotope exchange in vitreous silica" J. Appl. Phys., 50 (8), August 1979) have deliberately permeated low OH vitreous silica with deuterium (D.sub.2) (See: Hartwig, C. "Raman scattering from hydrogen and deuterium dissolved in silica as a function of pressure", J. Applied Phys., Vol. 47, page 956, 1976) and by the use of gamma or X-radiation or elevated temperature, ie, circa 500.degree. C. reacted the peroxy linkages contained therein with the molecular deuterium (D.sub.2) to produce deuteroxyl groups (OD). However, to thermally react the D.sub.2 molecules with the peroxy links in the vitreous silica to convert them to OD, a temperature greater than 200.degree. C. has heretofore been required. (See: Uesugi, N. et al. "Stress and temperature effects on optical loss increase for phosphar-doped silica fiber in the long wavelength region", Electron. Letters, Vol. 19, pp. 842, Sept. 29, 1983). Unfortunately such temperatures are usually too high for fibers mantled with organic polymers and for cables fabricated therefrom. Therefore a need exists to develop a technique for maximizing not only the deuterium permeation but also the reaction between D.sub.2 and the peroxy linkages which will allow universal application of the technique to vitreous silica optical fibers and cables made therefrom, irrespective of the materials with which such fibers and/or cables are clad.
Accordingly, it is a prime object of the present invention to provide high purity, low-OH vitreous silica which can be utilized to produce optical waveguide fibers which will not develop OH absorption bands within the transmission window of the fibers during normal use as well as in underground and underwater installations.
Another object of the present invention is to provide a new processing technique which creates optical waveguides from high purity low-OH vitreous silica which will not, in response to temperature changes, exposure to humidity and external forces, attenuate in their optical transmission properties because of a slowly increasing OH content.
Still another object of the present invention is to provide optical waveguide fibers from high-purity low-OH vitreous silica which achieve long term optical stability and permit long distance and transoceanic transmission of optical signals with minimal loss of absorption in the transmission window of 1.0 to 1.7 micrometers.
A further object of the present invention is to provide a novel and improved method of permeating vitreous silica with deuterium molecules at reasonable operating temperatures and pressures to create optical fibers, optical waveguides, and cables for optical transmission having long term optical stability within the transmission window.
A still further object of the present invention is to provide high purity low OH vitreous silica optical waveguide fibers which avoid the formation of additional OH over time in use by pretreatment with deuterium gas which permeates the fibers to react with the peroxy linkages therewithin to form an OD having an optical absorption band which lies at the end of the transmission window.
These and still further objects as shall hereinafter appear are achieved by the present invention in a remarkably unexpected fashion as can be readily discerned from a careful consideration of the following detailed description of certain exemplary embodiments thereof.